FIG. 1 (prior art) depicts a regulated cascode (RGC) amplifier 100, which has a relatively high output impedance r0 and a wide output voltage swing. RGC 100 includes a feedback amplifier 105, a bias voltage terminal VBIAS, a cascode circuit 110, an input terminal VIN, and an output terminal VOUT. Feedback amplifier a 105 includes a first current sourcing transistor 120 and amplifier transistor 125 connected in series between first and second power supply terminals VDD and VSS. Cascode circuit 110 includes input and output transistors 130 and 135 connected in series between output terminal VOUT and supply terminal VSS.
In operation, input transistor 130 converts input voltage VIN into a drain current I0 that flows through the drain-source path of output transistor 135 to output terminal VOUT. The drain-source voltage across transistor 130 should be relatively stable to suppress channel-length modulation that might otherwise reduce output impedance r0. The drain-source voltage of transistor 130 is therefore regulated about a fixed value by a feedback loop that includes amplifier transistor 125 and output transistor 135. Feedback amplifier 105 stabilizes the drain-source voltage of transistor 135 even when transistor 135 is biased in the linear region, which extends the usable range of the output signal VOUT.
FIG. 2 (prior art) depicts an improved regulated cascode amplifier (IRGC) 200. IRGC 200 is similar to a regulated cascode circuit 100 of FIG. 1, similar components having the same label and function. In addition to feedback amplifier circuit 105 and cascode circuit 110, IRGC 200 includes a level shifter 205. Level shifter 205 in turn includes a diode-connected transistor 210 and a second current-sourcing transistor 215 connected in series between supply terminal VDD and the drain of transistor 130. The gate and drain of transistor 215 connect to bias voltage terminal VBIAS and the drain of transistor 210, respectively. The gate and drain of transistor 210 connect to the gate of transistor 125, while the source connects to the drain of transistor 130.
The inclusion of level shifter 205 provides improved performance for low-voltage applications. Level shifter 205 limits the drain-source voltage VDS130 of transistor 130 to the difference between the gate-source voltage VGS125 of transistor 125 and the gate-source voltage VGS210 of transistor 210 (i.e., VDS130=VGS125−VGS210). This relatively low voltage at the drain of transistor 130 reduces the minimum level for output voltage VOUT.
The performance of IRGC 200 depends to a large extent on the characteristics of feedback amplifier 105, which in turn depends on the transconductance gM of transistor 125. A high gM, obtained by increasing the width W of transistor 125, improves the response time of feedback amplifier 105, a desirable characteristic for high-speed circuits. Increasing the width also reduces the gate-source voltage VGS125 of transistor 125, and consequently the drain-source voltage VDS130 across transistor 130. The relationship between the width of transistor 125 and the drain-source voltage VDS130 of transistor 130 sets an upper limit on the width of transistor 125: if the width of transistor 125 is too high, the drain-source voltage VDS130 of transistor 130 can be reduced to levels that bring transistor 130 into the linear range. This is undesirable, as the output resistance r0 of IRGC 200 varies considerably with output voltage VOUT when transistor 130 operates in the linear region. Unfortunately, the constraints on the width of transistor 125 limit the speed performance of IRCC 200 in low-voltage applications.